Surface-mediated four-wave mixing of nanostructures with counterpropagating surface plasmon polaritons.

We demonstrate that four-wave mixing (FWM) signals from individual Si nanoparticles can be generated by the surface fields of traveling surface plasmon polariton modes. We have chosen a counterpropagating excitation scheme in which the nanoparticle is exposed only to surface excitation fields and not to direct laser illumination. We show that background-free, surface-mediated FWM of nanoparticles can be acquired, and that the resulting nonlinear radiation is coherent.